Conventionally, there are known inkjet printers having print heads provided with a piezoelectric element (PZT).
For such print heads, a pulse voltage corresponding to image information is applied to the piezoelectric element, causing a predetermined distortion of the piezoelectric element. This distortion pressurizes ink inside an adjacent container, or ink channel, and an ink drop is discharged from the ink channel toward a print sheet, thus forming a printed ink dot. A plurality of printed ink dots yields an image on the print sheet.
In the inkjet printers described above, a distortion of a varied magnitude is generated in the piezoelectric element by varying the pulse amplitude (intensity) of the applied pulse voltage. The quantity of ink discharged from the nozzle is determined by the magnitude of the applied pulse voltage. Consequently, by adjusting the quantity of ink within an ink drop, a plurality of printed ink dot diameters can be obtained. For purposes of printing, varying ink dot diameters allows image tones to be expressed, for example, a large diameter ink drop expresses dark portions of an image, while a small diameter ink drop expresses light portions of an image.
FIGS. 13 and 14 are graphs showing examples of waveforms of a series of pulse voltages applied to piezoelectric elements of an inkjet printer print head for the purpose of discharging ink.
FIG. 13 illustrates a series of pulse voltages having a set amplitude from between 5 V to 50 V. Consequently, application of such pulse voltages to a print head, being constructed in a manner consistent with the above description, would result in ink drops having different volumes being discharged, wherein the volume of an ink drop is proportional to the pulse amplitude of its generating pulse.
Similarly, FIG. 14 illustrates a series of pulse voltages having a set amplitude from between 5 V to 50 V. Ink drops of different sizes, i.e., volumes, would be formed and discharged in accordance with these applied pulse amplitudes, thereby producing printed ink dots of different diameters on a print sheet. Note that, the rising portions of the 5 V and 10 V pulse voltages are steeper than the remaining pulse voltages. The steeper rising portions are used to suppress variations in printed ink dot shape by increasing the discharge velocities of the ink drops.
As shown in FIGS. 13 and 14, high amplitude voltage pulse voltages are necessary to provide a full range of printed ink dot diameters. Consequently, a power source, an amplifier, a switching IC, and so forth used for an electric circuit for generating such pulse voltages are required to have high voltage ratings, large capacities, and high responsivity. Understandably, these requirements cause increased production costs, increased circuit size, and the like.